Injection of solar energetic particles into both loop legs of a magnetic cloud

TL;DR

This study investigates the origin of bi-directional solar energetic particle distributions within a magnetic cloud, combining in situ measurements, modeling, and radio observations to determine injection sites and magnetic structure.

Contribution

It provides evidence that SEP injections occurred separately into both loop legs of the magnetic cloud, rather than by mirroring, using multi-instrument observations and modeling.

Findings

SEP injections occurred into both loop legs of the magnetic cloud

Bi-directional electron distributions are due to separate injections, not mirroring

Magnetic cloud structure and path lengths were characterized using models

Abstract

Each of the two STEREO spacecraft carries a SEPT Instrument which measures electrons and protons. Anisotropy observations are provided in four viewing directions. The SEP event on 7 Nov 2013 was observed by both STEREO spacecraft, which were longitudinally separated by 68{\deg} at that time. While STEREO A observed the expected characteristics of an SEP event at a well-connected position, STEREO B detected a very anisotropic bi-directional distribution of near-relativistic electrons and was situated inside a magnetic-cloud-like structure during the early phase of the event. We examine the source of the bi-directional SEP distribution at STEREO B. On the one hand this distribution could be caused by a double injection into both loop legs of the MC. On the other hand, a mirroring scenario where the incident beam is reflected in the opposite loop leg could be the reason. Furthermore, the energetic electron observations are used to probe the magnetic structure inside the magnetic cloud. We show that STEREO B was embedded in an MC-like structure ejected three days earlier. We apply a GCS model to the coronagraph observations from three viewpoints as well as the Global Magnetic Cloud model to the in situ measurements at STEREO B to determine the orientation and topology of the MC close to the Sun and at 1 AU. We also estimate the path lengths of the electrons propagating through the MC to estimate the amount of magnetic field line winding inside the structure. The relative intensity and timing of the energetic electron increases in the different SEPT telescopes at STEREO B strongly suggest that the bi-directional electron distribution is formed by SEP injections in both loop legs of the MC separately instead of by mirroring farther away beyond the STEREO orbit. Observations by the Nancay Radioheliograph of two distinct radio sources during the SEP injection further support the above scenario.